1. Field of the Invention:
The invention relates to a sensor for the detection of slurries, such as freshly mixed concrete or mortar, containing cement constituents, said sensor being particularly well adapted to differentiate between such slurries and water.
2. Description of the Prior Art:
When building a concrete structure of any kind, it is common practice to pour the freshly mixed concrete in between sections of wooden formwork, for example, or else in between such wooden formwork and the face of the excavated area itself or the panel walling erected to cover it. Moreover, in order to ensure the quality of the concrete work, it is also important to take particular care to ascertain the state of the pour itself when carrying out the compaction work. Unfortunately, however, the various sheathing and stop end boards used to contain the wet concrete also effectively prevent the operator from carrying out a visual check of the pour or, at least, make it particularly difficult to do so. The conventional method of checking the quality of the pour has thus been for an operator of some years experience to simply tap the outside of the form and to assess the state of the concrete inside from the sound and the response it makes. In other words, in the final analysis, it is the skilled operator's instinct that plays the major role in assessing the quality of the work. Again, the traditional methods used to compact the concrete after pouring normally involve either striking the formwork with a wooden mallet in those areas which are adjudged, on the basis of tapping the outside of said formwork, to have already been filled with concrete, or else by shaking the whole of the formwork with vibrators or by dipping a stick vibrator with water in order to encourage the freshly mixed concrete to settle. If the operator's judgment proves misguided, however, then there is a danger that striking or shaking may end up damaging a part of the formwork which has not yet been filled and this may in turn become a source of defects in the completed concrete work.
Clearly, therefore, the application of the operator's experience or instinct to the assessment of the state of a pour in cases where visual checking is out of the question can only be an imperfect technique at best. An error in the initial assessment may, therefore, result in the subsequent compaction operation being carried out to little or no avail, thereby leaving the completed concreting work defective. One additional on-site problem is, of course, that although the work of concrete placement is particularly strenuous, it is nevertheless, for the reasons outlined above, not well suited to being carried out by young and inexperienced workers.
In recent years, therefore, techniques involving the use of electrical sensors in the assessment of the quality of concrete placement work have been disclosed in, for example, the Japanese Open Gazette 63 (1988) - 101823 and the Patent Gazette 64 (1989) - 1621. These techniques involve the use of a pair of electrodes, one fitted to each side of a piece of formwork into which freshly mixed concrete is subsequently poured. The sensor works by virtue of the fact that an electric current can then be passed between the electrodes using the moisture in the concrete as a conductor.
The problem with this method, however, is quite simply that the detection element that is being used by the sensor is, in fact, water and not the freshly mixed concrete itself.
In other words, the efficacy of the sensor rests on the fact that, since freshly mixed concrete contains an element of moisture, the detection of moisture is deemed to indicate the presence of concrete. Unfortunately, however, the degree of dependability of this method of detection is not by any means absolute.
When wooden formwork is being prepared to accept a placement of fresh concrete, for example, it is always rinsed very thoroughly with water first. The object of this exercise is first of all to clean the surface of the formwork and, at the same time, to moisten the inner surface of the formwork with water in order to prevent it from absorbing the water from the concrete itself, which would have the effect of reducing the moisture content of the fresh concrete and, in so doing, of impairing the setting process. It is also quite possible that excess water may collect on the formwork itself as a result of rainfall or of seepage from the face of the excavation. Thus, while it may be possible to place some faith in the results produced by this type of electrical sensor in cases where no water is used at all, the fact that water is almost invariably present for one reason or another renders the detection of water virtually meaningless as a means of determining the presence or absence of concrete.
Another problem which needs to be considered is typified by the case where concrete is to be poured to form the walls of a tunnel. There is currently no way of obtaining real-time confirmation of the quality of the pour at the crown of the tunnel. By positioning the concrete sensor of the invention close to the crown of the tunnel in advance of the pour and then pouring the concrete, however, the quality of the pour can be confirmed electrically, thereby avoiding the risk of weakness that could result from an unsatisfactory pour.